<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>id选择器</title>
  <style>
    /* 定义 */
    #red {
      color: red;
    }
  </style>
</head>
<body>
  <!-- 使用 -->
  <div id="red">div 标签</div>
</body>
</html>